a thistle-like plant, Centaurea nigra, native to Europe and introduced into North America and New Zealand, that has reddish-purple flower heads: family Asteraceae (composites)
Also called: knapweed. See also centaury (sense 2)
hardheads in American English
a common knapweed, Centaurea nigra
See full dictionary entry for hardhead
Examples of 'hardheads' in a sentence
hardheads
Among the grass on chalky roadsides, knapweed, or hardheads, is opening.
Times, Sunday Times (2018)
The flowers of lesser knapweed, or hardheads, which are similar but less sprawling and are more like thistles, will follow.
Times, Sunday Times (2011)
The fields are full of colour in the summer, with grasses and flowers including meadowsweet, hardheads and buttercups.